Measurement / Quality Exclusives

Measurement: Styli Add the Touch of Precision to Probe Systems

August 29, 2011
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The correct choice and use of a stylus can help ensure consistent precision with a measuring system.

Stylus balls are made from ruby, silicon nitride, zirconia, ceramic or tungsten carbide, while materials for the holders and stems include titanium, tungsten carbide, stainless steel, ceramic and carbon fiber. To prevent stylus bending, it is recommended to use the shortest stylus possible, particularly with touch-trigger probe systems. Source: Renishaw

In machining terms, the end of the stylus is the tool tip when collecting measurement data with a probe. And with the proliferation of probe-based measurement devices-as well as the many different types of probes, such as kinematic, strain gage and scanning-an understanding of stylus designs, materials and applications can make the difference between consistent precision and frustration. In this overview, we’ll explain many of the most important considerations in choice and use of a stylus that can help ensure consistent precision with a measuring system.

Straight styli are the simplest and most commonly used. Straight shouldered stems and tapered stems are available. Styli with tapered stems offer better rigidity when the workpiece is easily accessible. The measuring travel direction should be near parallel to the part’s coordinate axes and at right angles to the work surface. Source: Renishaw

Stylus stem materials by application

For traditional kinematic inspection probes used on machine tools, ceramic stems and ruby ball styli are the first choice. Ceramic stems are lighter than tungsten carbide, have stiffness comparable to steel and are thermally stable-ideal for use in a harsh machine tool environment. For coordinate measuring machines (CMMs), steel and tungsten carbide are primary choices for shorter stems, with ceramic or carbon fiber recommended for longer stems.

For strain gage probes used on machine tools and CMMs, carbon fiber styli are recommended. Carbon fiber styli, whether hollow or solid, have low mass, making them best suited for use with strain gage probes. Carbon fiber also is the preferred material for long styli and extensions because it is stiff, light and thermally stable.

Styli for portable arm CMMs require robust design and construction and typically use a tungsten carbide stem with a Zirconia ball. The Grade 5 Zirconia balls are highly fracture-resistant. The balls are bonded to the high-strength tungsten carbide stem with impact resistant adhesive, and special construction techniques are used to ensure the joint is extremely rigid and virtually indestructible.

Scanning probes introduce other variables that affect the choice of ball materials.

Stylus ball considerations

The ruby ball is most commonly used for the stylus tip. It is exceptionally hard and smooth, with excellent compressive strength and abrasion resistance. Ruby balls are manufactured to various levels of precision defined by their grade, which relates to the maximum deviation of the ball from a perfect sphere.

Any error in sphericity will increase measurement uncertainty, and it is possible to lose as much as 10% of a CMM’s accuracy as a result of this error. It is important to remember that the more accurate a CMM is the more significant the effect of ball grade. The two most common ball specifications are grade 5 and 10 (the lower number is better). Grade 5 (sphericity of 0.13 micron) is ideal for most applications.

For higher accuracy CMMs, a grade 3 ball (with sphericity of just 0.08 micron) is recommended. Styli equipped with these balls utilize a special manufacturing technique because research has shown the form of the ball can be distorted by drilling and gluing-the standard way of mounting the ball on the stem.

The high-accuracy stylus uses an undrilled ball, bonded to a spherical cup. The amount of glue applied is critical to ensure maximum bond strength. A certificate of roundness is supplied with each of these styli, and they are coated with titanium nitride (gold color) for easy identification.

Ceramic hemispheric styli offer a large effective ball diameter with minimal weight. They are used for measuring deep features and bores, and are ideal for probing very rough surfaces where the ball diameter averages out the effects of the roughness. Source: Renishaw

Effects of scanning

Surface scanning with a probe results in a more aggressive type of interaction with the part that can ultimately affect the ball’s shape and accuracy, so it introduces a variable in the selection of ball materials.

Scanning produces three interactive phenomena: debris accumulation, adhesive wear and abrasive wear. Debris is practically unavoidable, and is independent of the stylus ball or part surface material. Debris can include metal oxide particles on the surface or airborne debris, such as coolant mist or paper dust. Debris can be removed with a dry lint-free cloth.

Adhesive wear involves the permanent transfer of material from one surface to the other through local welding or transfer of minute particles from one surface to the other. Adhered material cannot be removed with normal cleaning. Thus, as surface material from the workpiece adheres to the ball and remains in contact with the part surface, like materials will attract and further buildup can occur. Adhesion of material will eventually degrade the form of the stylus ball and compromise measuring results.

In the real world, significant measurement errors due to material adhesion are rare, and the amount of material transfer to the ball produces negligible change (<0.1 micron) in the ball’s form, based on a test scan of more than 350 meters.

For extreme scanning of aluminum parts, a silicon nitride ball is a better choice than ruby. Similar to ruby in its basic technical properties, silicon nitride is not attracted to aluminum.

Abrasive wear involves the removal of material from the ball or part surfaces. Based on extensive testing in scanning applications, ruby is the best ball material for most applications, including stainless steel and titanium. Silicon nitride can suffer abrasive wear on stainless steel or cast iron. Zirconia is the best choice for scanning cast iron parts.

Surprising variety of stylus types

Straight styli are the simplest and most frequently used type for measuring simple features where direct contact can be made with the part. However, alternative stylus designs are available for specialized applications, such as the star, swivel, disc and others.

Some of the more typical specialized styli include:

Star. Multi-tip star styli can be used to inspect a variety of different features where direct contact can be made with the part. Star styli allow efficient inspection of extreme points of internal features, such as slides or grooves in a bore, minimizing probe movement. Each tip on a star stylus requires qualification.

Pointer. While not appropriate for conventional X-Y probing, these styli are ideal for probing threaded forms, specific points and scribed lines. Radius-end pointer styli can be used to inspect the location of very small holes.

Ceramic hollow ball. These large styli are ideal for probing deep features and bores in X, Y and Z directions, and require datuming of only one ball. Probing with a ball stylus averages out the effect of very rough surfaces.

Disc. These styli are ideal for probing undercuts and grooves. Thinner discs require careful angular alignment to ensure correct contact with the part feature. A simple disc requires datuming on only one diameter (usually with a ring gage), but limits effective probing to only X and Y directions. Adding a radius-end roller allows Z-direction probing, providing the center of the radius end roller extends beyond the diameter of the probe.

Cylinder. These are ideal for probing holes in thin sheet material, threaded features and locating the center of tapped holes. Ball-ended cylinder styli allow full qualification and probing in X, Y and Z directions.

Custom design styli. Custom styli also are available for solutions to specific application requirements.

Star styli have rigidly mounted, multi-tip stylus arrangements, and can be configured with up to five styli on the stylus center. These configurations provide flexibility and enable contact with different features without changing the stylus. Source: Renishaw

Basics of stylus use

A few simple rules for stylus use can maximize accuracy for most probing applications:

Keep styli short and stiff. The minimum stylus length will give optimum results. A longer stylus amplifies errors and introduces the potential for excessive bending.

Minimize the number of joints. Joints and extensions introduce potential bending and deflection points.

Keep the stem diameter as large as possible. Thicker stems increase the stiffness of the stylus. If a stem with small diameter is required, use the longest possible M4 extension with a short stylus of the smaller diameter stem on the end.

Keep the stylus ball as large as possible. This maximizes the ball/stem clearance, reducing the chance of false triggers caused by “shanking out” on the stylus stem, and increasing the effective working length (measured from the center of the ball to the point at which the stem will shank against the feature). A larger ball also allows a larger stem diameter, increasing the stiffness of the stylus. Finally, the larger ruby ball reduces the influence of a part’s surface finish on measurement results.

Stylus extensions. Always pay close attention to the thermal characteristics of stylus extensions, which are available in steel, titanium, aluminum, ceramic and carbon fiber.

Deceptively simple...but not

Despite their apparent simplicity, there is a great deal of science-and certainly exceptional precision-behind the materials, designs and manufacturing techniques used to produce styli. Consider the basic stylus extension.

By combining a carbon fiber tube with titanium connecting components, the thermal coefficients of expansion of the two materials essentially cancel each other out. As temperatures increase, carbon fiber shrinks while titanium expands, so the stylus extension expands very little, even when seeing changes of 15 C to 40 C. Whether it is the method of attachment for the ball on the stem, or the combination of materials used, there is significant science behind the design, manufacture and use of styli. Q

Tech Tips

The ruby ball is most commonly used for the stylus tip. It is exceptionally hard and smooth, with excellent compressive strength and abrasion resistance.

The more accurate a CMM is the more significant the effect of ball grade.

Straight styli are the simplest and most frequently used type for measuring simple features where direct contact can be made with the part.

Calibrating styli

Before taking measurements, a probe must be calibrated for all measurement procedures by first probing a calibration sphere according to the machine and probe manufacturers’ instructions. Calibration requires use of a special program typically supplied with the coordinate measuring machine (CMM) or included with the prob-ing software used on the machine tool CNC (computer numerical control). The sphere is manufactured with exceptional precision and its exact dimensions are already established and input to the soft-ware. Calibration establishes the effective dimensions of the probe components. These values are stored in the CMM or machine tool control. If using more than one CMM, make sure to use a calibrated ball whose values have been input to the software on the given machine.

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